Improved RF Performance of Niobium Cavities via In-situ Vacuum Heat Treatment Technique

Vacuum thermal treatments (baking) are known to improve the superconducting properties of the RF surface layer of niobium cavities, and are employed as a last processing step to increase their efficiency determined by intrinsic quality factor Q0. A new method to perform the baking has been demonstrated. It consists in annealing of an evacuated cavity with the local heaters installed on its outer surface in a cryostat which ensures an exterior vacuum and protects the outer cavity surface from oxidation. Such a set-up has a number of advantages as it does not require to cool the cavity flanges during baking, and allows to perform the "cold" RF characterization of the cavity in situ, immediately after the thermal treatment without disassembly of heating elements. Moreover, the air exposure that causes partial degradation of Q0 by surface reoxidation is avoided. The heat treatment of a single-cell 1.3 GHz niobium cavity at 230 {\deg}C for 24 h demonstrated the doubling of Q0 at Eacc=10 MV/m (from 1.20e10 to 2.4e10) and retained the maximal accelerating field of 35 MV/m without quenching. The selection of treatment parameters is based on our previous XPS studies. This treatment ensures incomplete dissolution of the native oxide by oxygen diffusion, thereby preventing interaction of niobium surface with external contaminants. We propose to bake the cavities directly in a cryomodule, which would allow to use the treatment to improve their performance. The potential impact of material parameters on the components of surface resistance has been briefly examined.